Computational Fluid Dynamics Modeling of Short-Circuiting Prevention Mechanisms in Floating Treatment Wetland Retrofitted Stormwater Pond

Abstract Floating treatment wetland (FTW) is a nature-based solution for stormwater treatment. Its treatment efficiency predominantly depends on the fraction of inflow passing through the FTW root zone and residence time within the root zone. To prevent inflow bypassing the FTW, different short-circuiting prevention mechanisms (SPMs) have been deployed in field-scale studies. For example, full-width FTW (FWF) and FTW with impermeable curtains (FIC) were utilized to prevent flow short-circuiting in the horizontal direction. However, these SPMs are vulnerable to vertical flow short-circuiting and there is a potential for reduced residence time, which may affect the treatment efficiency. These vulnerabilities have never been studied and the effectiveness of the SPMs have not been compared previously. Furthermore, different parameters such as porosity of root zone, submergence depth ratio – SDR (length of root to water depth), position of FTW and coverage area have not been investigated for the SPMs, which have greater potential to influence the treatment efficiency of the SPMs. With an aim to investigate the effectiveness of SPMs, this study employed computational fluid dynamics (CFD) modeling in ANSYS Fluent, which was validated by hydraulic tracer experiments and subsequently simulations were conducted by the validated model. The results demonstrated that FWF and FIC had 14% and 8% higher treatment efficiency, respectively compared to a regular rectangular FTW (base case) at a non-dimensional removal rate, krtHRT= 20 (kr is the first-order removal rate of FTW and tHRT is the nominal hydraulic residence time of pond). An increase in treatment efficiency was found with increasing SDR due to larger volume of root zone. Treatment efficiency was augmented with declining porosity since a higher residence time at lower porosity (or higher vegetation density). FWF exhibited the highest treatment efficiency near outlet (> 60%), followed by near inlet (> 50%). Coverage area up to 30% with FWF was found to be optimum in this study.